See-Green Storage Vial
You know those little hard plastic cases that most plungers come in? They're great as original packaging, and for long term storage. But the downside is that they're airtight. If there is ANY moisture on your plunger and you close it up in one of those cases, you're asking for bacteria buildup or potential for mold. Personally I tend to keep mine in half of a scleral lens case (using it as a sort of pencil-holder) but that only allows for drying - it doesn't keep them protected or clean.
These vented cases offer a good alternative to allow plungers to stored without trapping moisture too.
Shipping instruction goes here